1. Field of the Invention
The invention relates to cigarettes, whose tobacco core is enveloped by a cigarette paper wrapping having areas of higher and lower permeability to air in the form of patterned, i.e, preferably circular zones.
2. Prior Art
Such cigarettes are already known from DE-OS No. 25 59 071, in which the cigarette paper has lower porosity zones in a range of up to 100 and higher porosity zones in a range of 150 to 2000, with an average porosity of 50 to 500 units, said units being determined in cm.sup.3 /min per 10 cm.sup.2 and at a pressure of 10 cm water column. In such known cigarettes the circular zones of alternating porosity are intended to bring about a controlled burning speed and/or an increased number of puffs.
In place of the porosity, DIN ISO/DIS No. 2965.2 gives the air permeability P as the air quantity in cm.sup.3 /minute, per cm.sup.2 and per kilopascal. It is calculated from the ratio of volume air flow in cm.sup.3 /min, which is traversed by the test pattern, to the product of the test surface of the test pattern in cm.sup.2 and the pressure difference between the two surfaces of the test pattern in kPa and used to be given as the so called "Coresta Value." The ISO/DIS No. 2965.2 was promulgated by the International Organization for Standardization and is incorporated herein by reference in its entirety. Thus, according to DE-OS No. 25 59 071 a cigarette paper is used, whose areas of lower porosity correspond to a permeability value of up to 10 P and preferably 5 P, whilst the areas of higher porosity have a value of 15 to 200 P, in the case of a total porosity of said cigarette paper of 5 to 50 P. On the basis of paper with high porosity, according to this reference the reduction of the porosity in the lower porosity areas can be brought about by applying gel-forming agents such as glue, methyl cellulose, gums or lacquers and varnishes. The cigarette papers indicated as being of low porosity therein and having a porosity of approximately 3.6 P can be perforated electrostatically or by pressure rollers or marking presses, so as to obtain higher porosity zones, the average porosity being approximately 24 P. The smouldering rates of such known cigarettes are e.g. in a range of 3.2 mm/min for a number of puffs of 9.7, whereas cigarettes with a conventional paper and an average porosity of approximately 26 P have a smouldering rate increased e.g. to 4.2 mm/min and a number of draws reduced e.g. to 7.5.
It is also known from DE-OS No. 23 15 613 to reduce the porosity of the paper by abrading its thickness, in order to increase the paper permeability or porosity. This allegedly makes it possible to favourably influence the flavour of the cigarette and the structure of the cigarette paper is not weakened by perforations.
It is also known from German Patent No. 17 61 500 to provide compressed regions in the form of a lattice or waffle-like pattern, e.g. a silicate slurry and to reinforce the intersection points with a non-combustible material, so as in particular to prevent the dropping of ash.
It is also known from U.S. Pat. No. 3,911,932 to use cigarette papers, whose porosity increases towards the tip, so that the smoke supply is more uniform.
To the extent that these known proposals are technically realizable, although they lead to certain advantages, the latter are obtained at the cost of other desired characteristics, because the essential parameters, such as the smouldering speeds of the tobacco and paper, air permeability of the cigarette paper, ventilation of the tobacco core and filter, tobacco quantity and moisture, as well as filling density and the geometrical construction of the cigarette influence in a very complex manner the smoking behaviour, the formation of side stream smoke and the number of puffs.
For example, in order to reduce the side stream smoke, the smouldering speed of the cigarette core must be reduced, because at lower smouldering speeds less tobacco is burned in the interval between puffs (according to DIN No. 10240=58 sec) and consequently less side stream smoke is produced. During puffing (according to DIN No. 10240=2 sec) roughly a standard tobacco quantity is burned, because the influence of the air to be drawn through the glow burning area dominates (according to DIN No. 10240=35 ml/2 sec).
In the case of normal smoulderable tobacco mixtures, the reduction of the smouldering rate of the cigarette core can be brought about by using low porosity paper, or normal porosity paper with smouldering rate-reducing additives. However, in the case of non-smoulderable, e.g. moister tobacco mixtures, a low smouldering rate can only be obtained through using highly smoulderable cigarette papers. However, the undesired side-effect of all these measures is a high number of puffs, because less tobacco is burned in the interval between puffs.